The fans of aircraft engines are designed to resist damage caused by foreign object ingestion. However, in certain circumstances, a fan may be damaged to such an extent that parts of one or more of the fan blades become detached from the rotor disk (referred to herein as a fan blade off event or FBO event). This may result in a significant imbalance requiring shutdown of the engine to minimise load transmission to the aircraft. The imbalance in the fan created by the blade loss generates extremely high radial loads which must at least be partially absorbed as the engine is run down to windmilling speed (i.e. the speed at which the rotor spins in a non-operative condition as a result of the aircraft moving through the air).
Under certain circumstances, the vibration resulting from the fan imbalance at windmilling speed can still be considerable. If not appropriately controlled, these vibrations may damage the engine structure and the aircraft and may result in difficulties to control the aircraft during approach.